The present invention relates generally to the finishing of paper and paperboard, and more particularly to an improvement in a waterbox calender for finishing paperboard to provide enhanced smoothness and gloss with a minimal loss of caliper.
It is common practice in the paper industry to add moisture to the paper web in the final finishing step at the machine calender during the papermaking process. This moisture application may take many different forms, but it is most commonly carried out by waterbox calendering. However, the use of a waterbox calender to impart a smooth surface to bleached board, or a similar substrate, has as an inherent problem, a lack of control of the amount of liquid picked up by the substrate in the calender nip. During conventional waterbox calendering, the amount of liquid supplied to the calender nip, is such that the nip is flooded. Thus the quantity of liquid picked up is determined by the calender roll diameter, operating speed, calender nip pressure, and substrate characteristics (thickness, sizing level, and roughness). Accordingly, the application of moisture to a substrate by use of a waterbox calender generally results in a transfer of liquid in excess of what is required to achieve the desired smoothness. The excess liquid weakens the substrate resulting in web breaks, and tends to establish a lower basis weight limit for production using a waterbox. For some applications, penetration of the excess water into the substrate also results in an undesirable reduction of the caliper of the web upon further calendering. In this connection, U.S. Pat. No. 2,130,530 to Fletcher, discloses the use of a typical waterbox calender in the manufacture of paper.
The amount of water picked up during waterbox calendering may be reduced by altering the substrate characteristics. This is typically done by: (1) reducing the surface roughness and void volume of the web by precalendering the substrate in one or more calender nips prior to the waterbox nip, or (2) by making the substrate less absorbent to moisture by over drying the web before waterbox calendering. However, both of these solutions are energy and labor intensive making them undesirable.
Other methods for adding moisture to the paper web at the machine calender include the application of steam and the use of water sprays. Application of steam onto a web to increase its moisture content is possible, but it requires that the web be cooled for efficient condensation of the steam. Moreover, in addition to the equipment required for cooling the web, it is difficult to condense the quantity of steam required to impart the same smoothness that can be achieved by waterbox finishing. The article entitled "Practical aspects of calender steam showers" by R. N Vyse and David J. Sawley, October 1988 TAPPI Journal, pp. 87-90, discloses the treatment of a paper web with steam before calendering. Spraying liquid directly onto a web, or onto a roll of a calender, is another method for increasing moisture content at the machine calender. However, spraying systems have limitations, primarily due to a lack of uniformity of application, and the production of wet streaks caused by overlap of sprays from adjacent nozzles, which results in nonuniform smoothness and caliper profiles. A third method for adding moisture to a web for machine calendering is to apply the liquid to the web before it enters the calender. However, this method like the use of steam, requires the addition of equipment to the papermachine prior to the calender stack. Thus while the use of a waterbox is generally agreed to be the preferred method for adding moisture to a paper web at a machine calender, the problems inherent with conventional waterbox calendering have yet to be solved. Accordingly it may be seen that a solution to these problems is desirable, and the solution offered by the method and apparatus herein represents a novel effort toward that end.